Microcontrollers have operated with two different power domains. Usually, a main power domain controls the operation of one or more secondary power domains. The main power domain powers a microcontroller, which then controls the powering up and powering down of the secondary power domains. Level shifters are used to shift input signal voltage levels between the different power domains. In some prior level shifters, a pair of cross coupled transistors forming an inverter are powered by the power domain to which signals are to be converted, and are controlled by the input signal and its complement.
If the microcontroller does not control the secondary power domains, it may not be aware that a secondary power domain has been powered down. This may result in floating input signal voltage levels and therefore an unpredictable output signal from the level shifter. Further, such floating input signal voltage levels may cause unnecessary current drain from the main power domain.